1. Field of the Invention
This invention relates to solid reducing agents such as those used in chlorination processes, and, in particular, to a method of activating solid carbon reductants.
2. Description of the Prior Art
The rate of carbon consumption is an important factor in a number of metallurgical processes. For example, in the production of anhydrous aluminum chloride, from aluminous raw material, as a precursor for producing aluminum, carbon or carbon monoxide is used as a reductant according to the following equations: EQU Al.sub.2 O.sub.3 +3/2C+3Cl.sub.2 =2 AlCl.sub.3 +3/2CO.sub.2 ( 1) EQU Al.sub.2 O.sub.3 +3CO+3Cl.sub.2 =2AlCl.sub.3 +3CO2 (2)
The rate of reaction (2) is significantly higher than reaction (1). However, the use of carbon monoxide as a reductant in reaction (2) requires the generation of carbon monoxide from carbon. Thus, for either chlorination reaction, the activation of carbon is desirable to both increase the rate and/or lower the reaction temperature. For the chlorination of aluminous materials, such as partially calcined alumina (PCA), metal grade alumina (MGA) and partially calcined aluminum chloride hexahydrate (ACH), an increased reaction rate at lower operating temperatures can be achieved with a highly activated carbon.
Petroleum coke is a known source of carbon reductants. Green petroleum coke, i.e., uncalcined coke, is known to have a moderate level of activity. However, a serious disadvantage is that it contains significant quantities of hydrogen and hydrocarbons which are chlorinated during the chlorination process. Not only does their presence result in increased consumption of expensive chlorine, but the hydrogen and hydrocarbons interfere with the chlorination reaction kinetics due to their vapor pressure over the surface of the solid reductant. Calcining coke drives off the hydrogen and hydrocarbons but to a great extent also deactivates the carbon. Fully calcined or "dead-burned" coke has very low activity insofar as chlorination reaction kinetics are concerned, but is essentially free of excess hydrogen and hydrocarbons. Accordingly, methods of activating partially and fully calcined coke have been sought after for some time.
Carbon is a highly refractory or inert material, that is, not readily susceptible to chemical attack. One substance known to attack carbon is an acid melt of aluminum chloride. By aluminum chloride melt is meant a mixture of salts, typically halide salts, containing greater than 50 mole percent aluminum chloride. Aluminum chloride itself sublimes rather than melts. Thus, in order to have aluminum chloride liquids, other salts which do melt, i.e., salts capable of maintaining a liquid state at atmospheric conditions, must be present.
It is also known that certain chloride salts, namely, the alkali chlorides, and to a lesser extent the alkaline earth chlorides and double chlorides such as sodium aluminum chloride, will catalyze reaction (1). For example, the chlorination reaction of "dead-burned" alumina (Al.sub.2 O.sub.3) with "dead-burned" carbon takes place at an exceeding slow rate. However, it is known that the addition of even a nominal amount of sodium chloride, e.g., 10 weight percent, will improve the kinetics and increase the reaction rate.
Despite knowledge of the catalytic effect of certain chloride salts upon reaction (1) and knowledge that acid melts are capable of attacking carbon, it has heretofore gone unrecognized that solid carbon particles including "dead-burned" carbon can be highly activated to an extent which renders them useful as reducing agents in processes such as that in reaction (1) by treatment with certain acid melts.